A tension leg platform, generally referred to as a TLP, is a type of marine structure having a buoyant hull secured to a foundation on the ocean floor by a set of tethers. A typical TLP is shown in FIG. 1 of the appended drawings. The tethers are each attached to the buoyant hull so that the hull is maintained at a significantly greater draft than it would assume if free floating. The resultant buoyant force of the hull exerts an upward loading on the tethers, maintaining them in tension. The tensioned tethers limit vertical motion of the hull, thus substantially restraining it from pitch, roll and heave motions induced by waves, currents and wind. However, unlike conventional platforms which are rigidly attached to the subsea floor, TLPs are not designed to resist horizontal forces induced by waves. Thus surge, sway and yaw motions are substantially unrestrained, and in these motions, a TLP behaves much like a conventional semisubmersible platform.
One problem presented by offshore hydrocarbon drilling and producing operations conducted from a TLP or other floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the platform at the ocean surface. This sealed fluid pathway is typically provided by a riser, which commonly takes the form of a substantially vertical, tubular element. In drilling operations, the drill string extends through a drilling riser, the drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a production riser is used to provide a pathway for the transmission of oil and gas to the work deck.
For TLPs and other floating platforms, special equipment known as a "riser tensioner" is required to maintain each riser within a range of safe operating tensions as the work deck moves relative to the upper portion of the riser. If a portion of the riser is permitted to go into compression, it could be damaged by buckling or by bending and colliding with adjacent risers. It is also necessary to ensure that the riser is not over-tensioned when the TLP hull moves to an extreme lateral position under extreme wave conditions or when ocean currents exert a significant side loading on the riser.
Most riser tensioners utilize hydraulically actuated cylinders with pneumatic pressure accumulators to provide the force necessary to maintain the upper portion of the riser within a preselected range of operating tensions. In one version, sheaves are attached to the buoyant drilling structure and tensioning cables are run over the sheaves and attached to the riser so that the riser is supported by one end of the tensioning cables. The other end of each tensioning cable is connected to a piston of an hydraulic cylinder. The hydraulic cylinders are connected to a relatively large accumulator which maintains the load applied by the cylinders at a relatively constant level over the full range of travel of the pistons. Thus, as the platform moves vertically, the pistons stroke to maintain a relatively constant upward loading on the riser. Typical of such a riser tensioner is that shown in U.S. Pat. No. 4,432,420, issued Feb. 21, 1984 to Gregory et al.
Another type of riser tensioner suitable for use on a TLP is described in U.S. Pat. No. 4,379,657 issued Apr. 12, 1983 to Widiner et al. Widiner also uses pneumatically pressurized fluid accumulators but eliminates the cables and sheaves used in earlier riser tensioners. Air and oil accumulators are connected to the cylinders to control the stroke of pistons. The piston rods are directly attached to a riser tensioning ring which supports the riser.
Both classes of riser tensioning systems described above rely on the use of hydraulic cylinders having sliding hydraulic seals. These seals have proven to be a troublesome maintenance item under offshore conditions. Damage to or failure of the seals can seriously degrade performance of the tensioner or render it altogether inoperative. These tensioning systems also require the use of hydraulic cylinders operating at pressures in excess of 6900 kPa (1000 psi). The use of a high pressure system presents several design and maintenance problems. It would be advantageous to provide a riser tensioner which avoids the need for sliding hydraulic seals and which has a lower operating pressure than those used heretofore.